# How to declare an operator with automatic argument bracketing

I'd like to have an easy way to declare an operator that brackets its argument with a particular delimiter. As an example I'd like to be able to write

\Pr{X}


to mean the same as

\operatorname{Pr}\left[X\right]


FYI I know how to do

\newcommand{\Pr}[1]{\operatorname{Pr}\left[#1\right]}


but I thought I once saw a package that provided some command like

\DeclareBracketedOperator{\Pr}{Pr}{[}{]}


sort of a combination of \DeclarePairedDelimiter and \DeclareMathOperator and I think it also defined a starred version of the operator that didn't take any argument and didn't produce the delimiters.

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I don't remember of any such package. Your \Pr command maybe obtained by

\usepackage{mathtools}
\DeclarePairedDelimiter{\Prfences}{[}{]}
\renewcommand{\Pr}{\operatorname{Pr}\Prfences}


With this definition, \Pr would behave exactly as if it was defined with \DeclarePairedDelimiter; that is, the *-form would use \left and \right.

If you want to always use \left and \right, you might follow Werner's suggestion, or go the hard way:

\makeatletter
\DeclareRobustCommand{\Pr}{\operatorname{Pr}\@ifstar\@firstofone\@Pr}
\newcommand{\@Pr}[1]{\left[#1\right]}
\makeatother


(requires amsmath, of course). Note: \renewcommand{\Pr}{...} is necessary, because \Pr is already defined in LaTeX.

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Here is a way to define your own operator with an optional bracketing:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{xparse}% http://ctan.org/pkg/xparse
\DeclareDocumentCommand{\Pr}{s m}{% \Pr[*]{..}
\operatorname{Pr}%
\IfBooleanTF{#1}% Condition on *
{#2}% Print only the argument in starred * version
{\left[#2\right]}% Print bracketed argument [ ] in unstarred version
}%
\begin{document}

This is $\Pr{2}$, and here is
$\Pr{\frac{\frac{a}{b+c}}{d-e}}=\Pr*{\frac{8}{9}}.$
\end{document}


The above code defines the macro \Pr{..} (actually redefines \Pr, since it is provided by amsmath), but also provides a starred * version. The unstarred version, as requested, prints its argument in extensible brackets, while the starred * version removes these brackets.

The xparse package provides an easy interface to specify starred versions of commands and environments. The xifthen package provides conditional support.

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